I once had to make a headboard for a photo shoot and found it surprisingly easy. I just ordered the wood from the local hardware store in the width and height that I needed (standard widths: twin: 38 inches; full: 54; queen: 60; king: 76). I then purchased some batting and fabric to cover the board and staple-gunned away like I was wrapping a gift.

Lately, I've been wanting a modern wood/no frills headboard, which set me on the path to these tree-derived DIY ones.
With some basic tools, these are easy to create and a lot cheaper than
ones you'd find in stores. Plus, you can modify size and color/stain to
whatever you suits your needs.